Transcript The Rosetta mission: results from the 2867 Steins

Asteroids 2867 Steins and 21 Lutetia:
results from groundbased observations
and from the Rosetta fly-bys
S. Fornasier1,2 , M.A. Barucci1,
M. Fulchignoni1,2
1) LESIA-Observatoire de Paris, CNRS, UPMC Univ
Paris 06, Univ. Paris-Diderot
2) Univ. Paris Diderot, Sorbonne Paris Cité
11 Sept. 2012
SONIA FORNASIER
1
2867 Steins
•Few information known when it was selected as target
Spectroscopy  Steins belongs to the rare E class: high albedo (0.4-0.6)
asteroids, are thought to be mostly differentiated and to have experienced
high heating episodes (T>1500K)
•Sharp band at 0.49 m due to sulfides like oldhamite
• Steins surface must be homogeneous (spectra at different rotational phases
are similar (Dotto et al., 2009, A&A 494, 24)
Fornasier et al. 2007, 2008
Barucci, Fulchignoni, Fornasier
et al., 2005, A&A , 430, 313
…enstatite achondrite (aubrite)
--- EL6 enstatite chondrite Atlanta
11 Sept. 2012
SONIA FORNASIER
2
2867 Steins: albedo
Polarimetry
Steins Polarimetric properties (slope, invertion angle and polarisation
values) are consistent with high albedo E-type asteroids (Fornasier et
al. 2006, A&A 449, 9)
Polarimetric slope gives an albedo of 0.450.1
Barucci et al., 2008, A&A 447, 665)
Radiometry
Obs. with SPITZER-IRS
R_effective= 2.46 ± 0.20 km
P(R) = 0.40 ± 0.07
(Lamy et al. 2008, A&A 487, 1187)
Steins emissivity spectrum is very similar
to that of the aubrite meteorites and of
the enstatite mineral (Barucci et al., 2008,
A&A 447, 665)
11 Sept. 2012
SONIA FORNASIER
3
21 Lutetia
Big asteroid initially classified as M-type (metallic) because of its visible colours
and moderate albedo values (0.18-0.22)
V+NIR spectrum not compatible with iron meteorites but with carbonaceous or
enstatite chondrites
Absorption bands at 0.43 μm (Belskaya et al, 2010, Lazzarin et al., 2004,2009) et
3 μm (Rivkin et al. 2000, Birlan et al. 2006,2010) associated to hydrated silicates
(Barucci et al. 2005, A&A 430, 313
11 Sept. 2012
SONIA FORNASIER
4
21 Lutetia
Polarimetry
Radiometry (SPITZER)
Pmin=-1.30±0.07%
inv=24.4±0.4 °
Large inversion angle is indicative of
• small particle size and/or
• high refractory material or inclusions
Belskaya et al., 2010
11 Sept. 2012
• I ≤ 30 JK−1 m−2 s−1/2  Lutetia is likely
covered by a thick regolith layer
(Lamy,et al. 2010)
•emissivity spectrum is different from
that of the iron meteorites and similar to
that of CV3/CO3
(Barucci et al., 2008)
SONIA FORNASIER
5
ROSETTA MISSION
2 fly-by with 2867 Steins (5 Sept. 2008) and 21 Lutetia (10 July 2010)
02/03/2004 2005-2007-2009
3/2007
09/2008
07/2010 67P : from 05/2014
OSIRIS
11 Sept. 2012
SONIA FORNASIER
6
ROSETTA – 2867 Steins Fly-by
Closest approach took place at 18:38:20 UTC on 5 Sept. 2008 at a minimum
distance of 803 km (80 m/px on WAC ; 60 % of the surface resolved during the
fly-by
• diamond shape with a big
crater (2.3 km) on the top
•Shape seems modelled by
YORP effects (r_eq = 2.65 km)
•Rotation retrograde, close to
perpendicular to the ecliptic
•No satellites found
7
Phase angle coverage: 0.27 < α< 141°
Geometric albedo = 0.40 ± 0.01
Phase function is typical for high albedo E type asteroids
8
The 2km-sized crater has well defined rim: relatively young?
•Surface age: very young 150—400 Myr
• shape modelled by YORP effects
• Large crater would have disrupt a monolithic body + YORP effect
Steins is probably a rubble pile asteroid
9
OSIRIS Spectrophotometry
Spectrophotomet
ry  E-type
Sharp 0.5 micron
band due to
sulfide (E[II])
The UV drop-off
shows that the
surface of Steins
is made by ironpoor minerals.
Keller et al. 2010, Science 327, 190
11 Sept. 2012
SONIA FORNASIER
10
Analysis of Steins’ surface composition
G-mode statistical method applied to the OSIRIS disk resolved images
reveal no significant variations with 95% of confidence level, confirming the
great homogeneity of the surface. This homogeneity seems to be consistent
with possible outcome of an impact that may have ejected the first layer of
the regolith on the whole surface
Leyrat et al. 2010)
11
ROSETTA – 21 Lutetia Fly-by
asteroid’s dimensions are
(126 ± 1) × (103 ± 1) × (95
± 13) km3
21 Lutetia fly by : 10 July 2010,
min distance 3160 km, best res. 60m/px
Irregular shape
Several big craters of (up
to 55 km)
Body shaped by
an extensive collision
history
11 Sept. 2012
SONIA FORNASIER
12
Surface
Geomorphology
Lutetia exposes
fascinating
morphology with
many craters,
grooves, ridges,
graben, scarps etc.
(Sierks et al. 2011)
Matteo Massironi, UPD
ROSETTA – 21 Lutetia Fly-by
•Surface age: Old (3.6Gyr)
and young regions (100 Myr)
landslides
• 237 boulders
identified of size > 100m
•Young" region shows craters completely
buried under the regolith blanket.
Blanket thickness: ~600 m (for d/D = 0.13)
11 Sept. 2012
SONIA FORNASIER
14
ROSETTA – 21 Lutetia Fly-by
(Sierks et al. 2011)
•Albedo = 0.19±0.010 @550 nm
• Albedo variations found, up to
30%
No absorption bands detected
the observed surface has no Ferich pyroxene / olivine, nor
hydrated minerals or organics
(Weiss et al. 2012)
OSIRIS+RSI data:
density = 3.4 ± 0.3 g/cm-3
LUTETIA may be a partially
differentiated object (metallic
core plus mantle composed
of carbonaceous material
(Weiss et al. 2012)
11 Sept. 2012
SONIA FORNASIER
15
21 Lutetia: summary
•
•
•
•
Asteroid fly-by was very successful
Lutetia is a piece of rock, too dense for a rubble pile
Old and young regions (10Ma-3.6Ga)
Blanket in North Pole region suggests a thick layer
of regolith/ejecta
• Lutetia exposes fascinating morphology with many
craters, grooves, ridges, graben, etc.
• OSIRIS spectrophotometry data agree with the ground
observations
with extension to 250nm
• Different from any other asteroid visited so far
• No satellites found, size limit 160m from WAC images
• No exosphere found (Na)
Thanks!
11 Sept. 2012
SONIA FORNASIER
17
Keller et al. 2010, Science 327, 190
Copyright: ESA ©2008 MPS for OSIRIS Team
MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA
NAC best res. Image (100m/px)
5Sept UT: 18:28, dist:5200 km,
phase=30°
WAC best res. Image (80m/px)
5Sept UT: 18:38:15
dist:806km,phase=50 °
18
Opposition Image
• NAC image @ phase angle
~0.15°, 16000 km far away from
Lutetia. 18 min before CA
• res. ~ 300m/px
• Albedo variation in
landslide area
• Line features are visible
• Small impact feature
UNRESOLVED SPECTROPHOTOMETRY
The Osiris data show a nice agreement with the groundbased spectrum, and
reveal a flat behaviour, without strong drop-off, in the UV region.